The invention relates to the field of optical inspection systems for inspecting semiconductor wafers, and more particularly to inspecting semiconductor wafers using a scanned beam of laser light.
Although the title of this description indicates multi beam scanning, it will be appreciated that the advances mentioned in some of the embodiments described below relate also to single beam scanning.
FIG. 1 shows a semiconductor wafer 10. Optical inspection systems are often used to inspect dies 20 on the semiconductor wafer 10. FIG. 2 shows several of the dies 20 of semiconductor wafer 10. Although it is certainly possible, and very common to compare the pattern of conductors of a die 20 with a reference image, it is also common for the comparison to be a die-to-die comparison. That is to say, a die 20 is compared with another die 20 instead of a reference image. For example, in a die-to-die comparison, dies adjacent to each other could be compared, such as the top to dies shown in the figure. Likewise, instead of comparing a die to another die in the same row, a comparison could be made between one die and another in the same column.
FIG. 3 shows a beam of light 100 being made to impinge upon the surface of the die 20. The main optical scanning direction is indicated by the letter O. The mechanical scanning direction is indicated by the letter M. The mechanical scanning direction is the direction in which the stage moves the wafer, and also may be referred to as the direction of movement of the wafer or the target movement direction.
FIG. 4 shows a system that employs dark field imaging. In FIG. 4, the wafer 10 is mounted on the X-Y stage 12. A light source 200 produces light which is shaped, focused, or operated on as necessary by optics 210 and provided to a scanner 300. The scanner 300 outputs the light in such a manner that, after passing through optics 310, follows a scan pattern across the die 20 on the wafer 10. The scanner can be implemented by a rotating polygon, deflection mirror, or acoustic-optics deflector (AOD). As is well known, dark field imaging uses detectors 410 positioned so as to capture light 110 that is scattered rather then reflected off of the surface of the die.
FIG. 5 shows a system that employs bright field imaging. In bright field imaging, the reflected light 120 is captured by a detector 420. The detector can be a PIN diode, PMT, or line CCD camera. The optics 310, in this case, could include e.g. a beam splitter. Thus, light from the light source 200 travels through beam shaper 205 and optics 210, and is caused by the scanner 300 to impinge on the surface of the die 20 on wafer 10, and the reflected light 120 is channeled back through the optics 310 to the bright field detector 420.
To put it another way, the bright field optical inspection system collects the specularly reflected light, whereas the dark field system collects the scattered light. Usually, a bright field system is used with very high-resolution imaging optics, and the inspection of the wafer is performed in such a manner that the pixel size is very small. The small pixel size makes maximum advantage of the high-resolution imaging optics and the large amount of reflected light. Bright field systems thus provide a great deal of detail, and are excellent when such detail is necessary.
Dark field systems provide a much higher throughput. Dark field systems typically use laser scan technology for illumination, but the inspection of the wafer is usually performed in such a manner that the pixel size is relatively large. The use of scattered light detection is advantageous in that it has a high signal to noise ratio and even relatively small defects can be detected with high throughput. As will be understood, higher throughput can be obtained with relatively lower, data rate.
It will also be appreciated that the systems of FIGS. 4 and 5 could be combined, resulting in a system having both bright field detectors and dark field detectors, positioned appropriately so that the scattered light can be detected by the dark field detectors and the reflected light can be detected by the bright field detectors as accomplished in Applied Materials wafer inspection tool, Compass™. It is important to notice that in such a configuration the high throughput is obtained by a scanning spot with a relatively large pixel, so the resolution of the BF detector is relatively poor.
What is needed is a better approach that provides higher throughput with also high resolution in bright field mode.